Efficient oil field production requires the recoverable reserves of a reservoir to be estimated periodically. Current practice in reservoir engineering relies heavily on measurement of saturation-dependent relationships of relative permeabilities and capillary pressures. Relative permeabilities combined with absolute permeability reflect flow capacity for a fluid phase for a given pressure drop, while capillary pressure refers to the difference in pressures between two fluids at equilibrium saturations.
Various techniques have been available prior to the present invention for measuring the capillary properties of small core samples. One is the centrifuge technique by which the denser of two phases is drained from a core sample by centrifuging the sample until no further fluid is collected, and repeating this procedure for successively higher accelerations. The capillary pressure and average saturation at each point are computed from the density difference of the fluids, the dimensions of the core and centrifuge, the amount of fluid displaced and the speed of rotation. This procedure provides substantial data about the capillary properties of the sample, but it is quite time consuming, requiring weeks to complete the experiments.
The "restored-state" method of measuring capillary pressure utilizes a porous disc saturated with the wetting fluid which remains impermeable to the non-wetting fluid at the pressures encountered during the test. Wetting fluid is then displaced from the core through the disc by maintaining a constant pressure in the non-wetting phase at the inlet face of the core. A constant pressure is maintained at the outlet face of the disc at a value less than at the inlet. The core is brought to capillary equilibrium when the flow rate ceases and the pressure and saturation distributions of the core sample become uniform. The saturation of the sample is then measured and the difference in pressure between the two phases is the capillary pressure at that saturation. This procedure is repeated at higher inlet pressures, with each stage giving an additional point on the capillary pressure curve, until the desired amount of data is obtained. While this test procedure provides valuable data, it is also time consuming.
While the lengthy procedures of these tests can be justified in order to measure the entire capillary pressure relationship of a sample, it would be desirable to be able to employ a much faster test procedure for the purpose of obtaining information which would allow interpretation of gravity drainage experiments in short core samples, which are typical of cored reservoir rock available for laboratory studies. For this purpose a determination of the threshold and critical capillary pressures is very useful, particularly if these points can be determined at low gas saturations. Test procedures for accurately and rapidly determining these values under the conditions required have not been known, however, prior to the present invention.